Media for finishing plastics and soft metals

ABSTRACT

FINISHING MEDIA FOR MECHANICAL BARREL OR VIBRATORY FINISHING OF SOFT METALS AND PLASTICS CONSISTING OF DISCRETE FLEXIBLE POLYMERIC SHAPES HAVING A SHORE D HARDNESS OF 45 TO 80 AT ROOM TEMPERATURE, THE HARDNESS OF SAID SHAPES BEING INVERSELY VARIABLE WITH TEMPERATURE, PREFERABLY FORMED FROM A RESILIENT POLYESTER RESIN CONTAINING FINELY DIVIDED ABRASIVE HAVING PARTICLE DIAMETERS BELOW 30 MICRONS, AND METHOD OF FINISHING SOFT ARTICLES, INCLUDING THE STEPS OF FIRST AGITATING IN THE PRESENCE OF MEDIA AT A LOW TEMPERATURE WHEREBY INCREASED HARDNESS OF MEDIA INCREASES METAL CUT RATE AND SUBSEQUENTLY RAISING THE TEMPERATURE WHEREBY THE METAL CUT RATE IS DECREASED BUT THE SURFACE SMOOTHNESS PRODUCED ON ARTICLES IS GREATLY INCREASED, PLASTICS BEING FINISHED IN COLD WATER TO INCREASE CUT RATE AND MINIMIZE SOFTENING AND SWELLING.

Oct. 19, 1971 J. a. KITTREDGE ET 3,613,317

MEDIA FOR FINISHING PLASTICS AND SOFT METALS flriginal Filed May 26,1967 I N VE N T( )RS JohmfifK/rmzwaE lQ/C'HHRD L. 4250 United Stateslatent @fice 3,613,317 MEDIA FOR FINISHING PLASTICS AND SOFT METALS JohnB. Kittredge and Richard L. Larson, White Bear Lake, Minn., assignors toMinnesota Mining and Manufacturing Company, St. Paul, Minn.

Original application May 26, 1967, Ser. No. 641,666, now Patent No.3,504,124, dated Mar. 31, 1970. Divided and this application June 25,1969, Ser. No. 869,980

Int. Cl. B24b 31/02 US. Cl. 51-164.5 3 Claims ABSTRACT OF THE DISCLOSUREFinishing media for mechanical barrel or vibratory finishing of softmetals and plastics consisting of discrete flexible polymeric shapeshaving a Shore D hardness of 45 to 80 at room temperature, the hardnessof said shapes being inversely variable with temperature, preferablyformed from a resilient polyester resin containing finely dividedabrasive having particle diameters below 30 microns, and method offinishing soft articles, including the steps of first agitating in thepresence ofi media at a low temperature whereby increased hardness ofmedia increases metal cut rate and subsequently raising the temperaturewhereby the metal cut rate is decreased but the surface smoothnessproduced on articles is greatly increased, plastics being finished incold water to increase cut rate and minimize softening and swelling.

This is a division of application Ser. No. 641,666 filed May 26, 1967,now Pat. No. 3,504,124 dated Mar. 31, 1970.

This invention relates to media for industrial barrel or vibratoryfinishing. More particularly, the invention relates to flexiblepolymeric media having the capability of producing highly smoothsurfaces on soft metal and plastic articles.

The finishing of polymeric plastics and softer metals, such as zinc diecastings, brasses, copper, and aluminum alloys, is commerciallyaccomplished by either hand or semi-automated bufiing, which isobjectionable due to high direct labor costs and the dirtiness of theoperation. A second commercially-used method involves mechanicalfinishing in rotating barrels or vibrating tubs in the presence of mediawhich usually consist of natural stones, fine abrasives bound togetherby rigid resins, or fused ceramics. Previously, synthetic media havebeen formed from hard ceramics and plastics, i.e. those having a Shore Dhardness of 90 or more, because high hardness was believed necessary toreduce the wear rate of the media. Plastic media in the past whichcontained no abrasive have not proved suitable for cutting (i.e.,removal of significant amounts of surface material from the parts beingfinished) soft materials such as plastics; externally added abrasivesare rapidly (flushed away by the large quantities of water required forkeeping the parts clean during the finishing operation. In vibratoryfinishing equipment the use of such external abrasives is in any case acumbersome, messy, and costly added step.

Insofar as we are aware, abrasive-filled plastic media have in partalways contained abrasives ofi 100 microns or more in diameter and/orhave been formed from very hard, rigid binders. Such media have beenfound in themselves to impinge and dent articles made from softmaterials. Thus media such as wood, corncobs, and the like, have beenused for forming smooth finishes on soft materials. The latter media,however, provide such a slow rate of finishing that the tumbling orvibration must be carried on for a period of several days, thusobjectionably tying up the finishing equipment and the parts beingfinished.

3,613,317 Patented Oct. 19, 1971 In accordancewith the presentinvention, media are provided which enable the formation of a fine,highly smooth surface finish on soft articles with greatly increasedspeed and economy. The media of this invention, further, have atemperature variable hardness such that the rate of abrasion and thesmoothness of the surface produced can be varied by adjusting thetemperature of the finishing operation. The media of this invention arepreferably used by vibrating the media and articles to be finished inwater.

The finishing operation of this invention can be carried out initiallyat at low temperature, which increases the hardness of the media,thereby in turn increasing the cut rate of material from the surface of}the parts being finished. The finishing operation of this inventionwherein the cut rate is controlled by altering the temperature, can

be carried out using media having coarser abrasives than microns, ifdesired, so long as the desired decrease in hardness with increasingtemperature occurs. As the finishing operation progresses, thetemperature of the water can be increased, thereby softening the media,consequently improving the smoothness of the surface produced, whilereducing the cut rate. Thus, both rough and fine finishing cuts, whichformerly had to be conducted in the presence of two separate media, can,as a result of this invention, be carried out in the same agitatorwithout need for removing the parts and exchanging one media foranother. The economics resulting from this degree of control provided bythe process of this invention will readily be apparent to those skilledin the art.

Briefly summarized, the media of this invention are organic polymericshapes containing at least 20% by weight of an abrasive filler. Theshapes have a Shore D durometer hardness (initial reading) at roomtemperature between about 45 and 80. Preferably the hardness of themedia decreases substantially, as the temperature is progressivelyincreased. Preferably the media decrease to a Shore D hardness belowabout 35 if the temperature is increased to 212 F. The abrasive fillerused has a hardness greater than 4 on the Mohs scale, and is essentiallyfree of abrasive particles greater than 30 microns in diameter.,Preferably the abrasive has an average particle diameter below 15microns. While the finishing operation can, broadly, be carried out inwater at temperatures ranging from 32 to 212 F., finishing is preferablyinitially carried out at a temperature in the range of about 35. to 50F., and subsequently at a raised temperature of; about to F. in the caseof metal finishing. In the case of plastics it is preferred to keep thearticles below about 50 F. in order to avoid undue softening of theplastic articles being finished, The media of this invention are in asize range such that they have a maximum dimension of about A; to 2inches (0.32 to 5.1 cm.).

The finishing of soft plastics, for example cellulose acetate orcellulose propionate used in eye glass frames, buttons, etc., haspresented particular finishing problems, which, prior to this invention,resulted in costly finishing techniques. Presently such materials arefinished dry in rotating barrels using wood pegs and an added looseabrasive. The rough cutting operation is typically carried on for 20 to50 hours using a coarse added abrasive, and is followed by a refiningcut of 20 to 40 hours using a finer added abrasive. Thus, a total of 40to 90 hours of finishing is required to finish the articles to the pointwhere they are ready for final polishing. The finishing time employed inany given case in directly related to the condition of the articles atthe outset of the finishing operation and to the final qualityrequirements. In one embodiment of this invention the finishing ofmolded plastic materials is carried out in vibratory equipment in coldwater, i.e., at temperatures of 32 to 50 F., and preferably 35 to 45 F.These low temperature appear to greatly reduce swelling and softening ofthe plastics due to water absorption, as compared to warm or even roomtemperature water, and appear to greatly reduce the finishing time bymaking the parts more brittle and more easily abraded. In accordancewith this invention, the rought cutting can be accomplished in coldwater in from 2 to hours in the presence of rough cutting media, whichare a rigid plastic binder containing abrasives having average particlediameters below 20 microns and preferably below about microns. The roughcutting operation is followed by a refining operation using theflexible, abrasive-containing media of this invention as defined herein.For the refining step the flexible media must have a hardness at thefinshing temperature less than that of the plastic being finished. Therefining step of this invention can be carried out in as short a time asone to 5 hours in cold water in a vibrator. Thus, a greatly reducedfinishing time of 3 to 15 hours is required to prepare plastic articlesfor final finishing, with results comparable to those obtained in the 40to 90 hours required in the currently-used dry tumbling processes. Finalpolishing can be carried out in accordance with techniques known tothose skilled in the art. The refining step using flexible mediacontaining fine abrasive in accordance with this invention is criticalto achieving the low finishing time and excellent surface finishobtainable by the cold-water plastic finishing method of this invention.

Insofar as we are aware, abrasive-containing polymeric media when usedin vibratory equipment has not, prior to this invention, been capable ofreducing the roughness height on the surfaces of soft metal (i.e.,softer than steel) articles being finished to a value better than about5 to 10 microinches. Roughness height is understood in the art to be thearithmetical average (a.a.) or root means square (RMS) deviation fromthe nominal surface, measured normal to the nominal surface, inmicroinches, i.e. millionths of an inch. All measurements given here arein microinches, RMS. The media of this invention make possible highlysmooth finishes in the range of one to 5 microinches (.025 to 0.13micro).

The invention will be further explained with reference to theaccompanying drawings, wherein:

FIG. 1 is a diagrammatic view cross sectional illusstrating the processof this invention, and

FIG. 2 is a perspective view of a typical shape used for a medium ofthis invention.

Referring more particularly to FIG. 1, there is seen, diagrammatically,a preferred maner of practicing the invention, using appaartus of thevibratory type. Tub 10 or similar receptacle is fixed for vibration onmounting means 12. Practicing the method of this invention, thereceptacle 10, articles being worked on 14, media 16, and usually water,are subjected to controlled gyratory vibration. The constant agitationof the parts and medium produces a scrubbing action that operates on allportions of the work load simultaneously. The motion of t he vibratorcontents is indicated generally by arrows 18.

In FIG. 2 is shown a shape of a preferred type for use in practicing theinvention. A conical shape shown in FIG. 2 can be formed by dropwisedeposition of a viscous plastic on a receiving surface, or by othermolding processes. The media of this invention can be made in variousother shapes, such as spherical, cylindrical, oval, etc. The conicalshape shown in FIG. 2, however has been found to provide a verydesirable ability to finish the inside of cutout parts without lodgingtherein, and has thus been found to be particularly suitable.

As noted above, the finishing operation is preferably carried on inwater, both to provide the desired ease of control of the finishingtemperature, and to insure thorough flushing of abraded material fromthe parts.

As noted above, the media of this invention should have a hardness atroom temperature of about 45 to 80. Preferably the media softengradually as the temperature is increased to a hardness in the range ofto at a temperature below 212 F t The preferred resins for forming mediaof this invention are materials which can be mixed in solvent-freeliquid form and reacted Within a short period of time to formcross-linked polymers having the desired hardness. Examples of thepreferred resins are polyesters, polyurethanes, and epoxy resins. Thepreferred polymers are sufliciently soft and deformable, so that dentingof the parts being finished due to impingement by the media issubstantially eliminated.

The abrasive used in preparing media of this invention for producingextremely smooth surfaces contain abrasive particles less than 30microns in diameter, as previously noted. The abrasive should have ahardness on the Mohs scale of over 4, and preferably 6 to 9. Examples ofsuitable abrasives are quartz, silicon carbide, alumina, flint andemery.

The media and methods of this invention are preferably carried out invibratory as contrasted with simple tumbling type equipment, because ofthe increased speed of finishing obtainable. The vibrator may have a tubof either circular or toroidal cross section, both of which types arecommercially available. Preferably the tub is fixed for vibration ofabout /a to inch in the vertical direction. Vibration of the units isgenerally produced by mechanical means such as an eccentric weight or byelectromechanical means.

The invention will be further illustrated by the following examples, inwhich all parts are given by weight unless otherwise indicated.

EXAMPLE I A thick highly viscous liquid reaction mixture was prepared bymixing 67 parts of unsaturated isophthalic polyester (Corezyn 3), 33parts of styrene monomer, and 3 parts of benzoyl peroxide paste (thepaste being formed from 50% benzoyl peroxide, 50% butyl benzylphthalate, plasticizer), and 0.2 part of a 6% solution of cobaltnaphthenate in mineral spirits, and 0.2 part red pigment. 90 parts offinely divided quartz abrasive less than 15 microns in diameter wereadded to the mixture. Immediately after mixing, the abrasive-containingresin was cast into the form of conically shaped particles. The mediaparticles were cured in an oven at 225 F. (107 C.) for about 20 minutes.The media had a Shore D hardness at room temperature of 55 (initialreading).

Approximately 22 /2 cubic feet (57 to 71 liters) of media of mixedsizes, selected to approximate those obtaining in a commercial operationin which large media are periodically added to those which have beenabraded to a smaller size, together with about /2 cubic ft. (14 liters)of cellulose acetate plastic eye glass frames were placed in a vibratorhaving a 3 cu. ft. liters) toroidal shaped tub. 78% of the media werecones 1% inches in diameter and 1% inches high, 17% were inch by inch(1.90 cm.), and 5% were /2 inch by /2 inch (1.27 cm.). The eye glassframes had previously been rough cut to remove large surface blemishesand flash from the molding operation. The tub was vibrated vigorously atA1 inch (.64 cm.) amplitude for 2 hours. Cold water at 45 F. (7 C.) wasfed through the vibrator at the rate of 60 gallons per hour (170 litersper hour). The eye glass frames were then ready for final polishing.

EXAMPLE II Conical media were made using the same compositions andfollowing the procedure set forth in Example I, with the exception thatparts of -micron quartz abrasive was substituted for the 15-micronquartz. These media also had a room temperature Shore D hardness(initial reading) of 45. The media were used for finishing cast zinc,using water of varying temperatures. The zinc parts had a smoothsurface, initially, with a surface roughness height of 2-4 microinches.The media (using the same blend of sizes as in Example I) and zinc partswere vibrated for about 1 hour in water at various temperatures. Afterone hour of vibration at 40 F. (4 C.) a part had a surface roughnessheight of about 20 mieroinches,

RMS (.51 micron). The media at this temperature had a Shore D hardnessof 60 and 0.074% of the metal was removed. Another part of the same sizeand shape was vibrated at 75 F. (24 C.) for one hour, at the end ofwhich the surface roughness height was 13 microinches, RMS (.33 micron).The media Shore D hardness at this temperature was 45, and the metalweight loss was .046%. A third part was vibrated at 110 F. (44 C.). Atthis temperature the media had a Shore D hardness of 35. This part hadan average surface roughness height of about 8 microinches (.20 micron)and .030% of the metal was removed after one hour. This exampleillustrates the effectiveness of the media of this invention in varyingthe smoothness of the finish and metal removal rate by varying thefinishing temperature.

EXAMPLE HI Two medias were prepared according to the proceduresspecified in Example I except that both were all of the /1 x /4 inchsize. The first of these, Media A, was identical in composition to thatdescribed in Example I. The second, Media B, was identical in allrespects except that the quartz abrasive was essentially 5 microns indiameter or finer. A comparison was made between Media A, Media B, andhand bufiing zinc die castings and the luster of these surfaces aftercommercial copper-nickelchromium plating.

Three zinc die castings having an essentially flat surface about 1-%inches square (4.5 centimeters square) were belt sanded flat andpolished by buffing to about 1-2 microinches, RMS (.025 to .051 micron).One of these was placed, with Media A, in a tub-type vibrator andprocessed wet for 1 hour. One of the others was processed with Media Bfor one hour. Surface finish was determined, and the parts were platedwith 0.2 mil (5.1 micron) copper, nickel and 0.01 mil (0.25 micron)chromium. Nickel thickness varied as indicated. Results are given inTable I.

TAB LE I Dct reflectivity is the distance a 1/64 (0.40 mm.) dia. whitedot,

outlined in blue, will maintain its clarity. The dot can be seen 70inches (178 cm.) from a good quality mirror, for example. These datashow that Media B is equal to bufling in developing luster after thecopper-nickel-chromium plating with .3 mil (7.6 microns) nickel.Further, these data show a heavier nickel deposit is required (.5 mil)(12.7 microns) before the Media A processed part is comparable inluster.

Two zinc die castings were processed by vibratory finishing using apolymeric media having a Shore D durometer hardness of 90 containingmicron and finer abrasive particles for a time sufiicient to reduce thesurface finish to 7-8 microinches, RMS (0.18 to 0.20 micron). Bothcastings were then vibrated for one hour in Media A. The parts afterthis processing had a surface finish of 5 microinches (0.13 micron) RMS.One of the parts was plated with .2 mil (5.1 micron) copper, .3 mil (7.6micron) nickel, and .01 mil (0.25 micron) chromium. The plated part hada slightly hazy finish. The other casting was subsequently given anadditional hour of processing in Media B. This second part had a surfacefinish of 3 microinches (.076 micron), RMS. After plating with copper,nickel and chromium to the same thicknesses as the first part, this parthad a smooth shiny surface with excellent reflectivity.

EXAMPLE IV (A) Part A The following materials were mixed and degassed 2hours at 60 C. and 2 mm. pressure in a vacuum oven:

Parts Castor oil (anhydrous grade) 200.0 Phenylmercuric acetate(catalyst) 0.6 SiO 30 micron and smaller 268.4

(B) Part B Toluene diisocyanate. (:20 mixture of 2,4 and 2.6 isomers)71.5 Polyoxypropylene triol, 400 molecular weight (C) Castings 100 gramsof Part A were mixed with 21.0 grams of Part B. The reaction mixturecontained -NCO and --OH groups in a ratio of 1.03 to 1, and containedparts silica per grams of polymer. The fluid mixture was poured at roomtemperature into pan molds of the desired shape. The shapes were curedat 65 C. for about 15 minutes. The shapes were found to have a Shore Dhardness at room temperature of 45. At F. the shapes had a Shore Dhardness of 35.

We claim:

1. Tumbling media capable of forming a smooth surface finish on plasticor soft metal articles comprising discrete flexible organic polymericshapes having a Shore D hardness between 45 and 80 at room temperature,which hardness decreases to a value below about 35 at 212 F., saidshapes containing at least 20% by weight of an abrasive filler having ahardness greater than 4 on the Mohs scale, and being essentially free ofabrasive particles greater than 30 microns in diameter.

2. Media according to claim 1 wherein said organic polymeric materialhas a glass transition temperature below room temperature and has ahardness continuously inversely variable with temperature in the rangeof 32 F. and 212 F.

3. Media according to claim 1 wherein said polymeric material is apolyester resin which contains abrasive particles having an averagediameter below 15 microns.

References Cited UNITED STATES PATENTS 2,328,998 9/1943 Radford 51-1645X HAROLD D. WHITEHEAD, Primary Examiner Patent No, 3,613,317 DatedOctober 19, 1971 John B1 Kittredge et a1 Inventor-(s) It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

In the heading to the printed specification, lines 5 and 6, "Assignorsto Minnesota Mining and Manufacturing Company, St. Paul, Minn." shouldread Assignors, by mesne assignments to The Wheelabrator Corporation,Mishawaka, Ind. a corporation of Delaware Signed and sealed this 10thday of October 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents RM 30-1050 ($69) USCOMM-DC eoammuo 9 U 5 GOVERNMENT PRINTINGOFFICE 1969 OJ6633L

